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Altair star system
Altair is a type-A main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity.34 Altair possesses an extremely rapid rate of rotation; it has a rotational period of approximately 9 hours.4 For comparison, the equator of the Sun requires just over 25 days for a complete rotation. This rapid rotation forces Altair to be oblate; its equatorial diameter is over 20 percent greater than its polar diameter. Oblateness and surface temperature False-color image of the rapidly rotating star Altair, made with the MIRC imager on the CHARA array on Mt. Wilson.The angular diameter of Altair was measured interferometrically by R. Hanbury Brown and his coworkers at Narrabri Observatory in the 1960s. They found a diameter of 3 milliarcseconds.13 Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Altair was later observed to be flattened by infrared interferometric measurements made by the Palomar Testbed Interferometer in 1999 and 2000. This work was published by G. T. van Belle and his co-authors in 2001.8 Theory predicts that, owing to Altair's rapid rotation, its surface gravity and effective temperature should be lower at the equator, making the equator less luminous than the poles. This phenomenon, known as gravity darkening or the von Zeipel effect, was confirmed for Altair by measurements made by the Navy Prototype Optical Interferometer in 2001, and analyzed by Ohishi et al. (2004) and Peterson et al. (2006).414 Also, A. Domiciano de Souza et al. (2005) verified gravity darkening using the measurements made by the Palomar and Navy interferometers, together with new measurements made by the VINCI instrument at the VLTI.15 Altair is one of the few stars for which a direct image has been obtained.16 In 2006 and 2007, J. D. Monnier and his coworkers produced an image of Altair's surface from 2006 infrared observations made with the MIRC instrument on the CHARA array interferometer; this was the first time the surface of any main-sequence star, apart from the Sun, had been imaged.16 The false-color image was published in 2007 and can be seen above and to the left; the brighter regions are shown in white and darker regions in blue. In this image, North (the direction towards the North Celestial Pole) is up and East is left, and the white line is the rotational axis of Altair. The black grid shows lines of latitude and longitude in an Altair-centric coordinate system. The von Zeipel effect can be observed in the image, which shows a white spot near the pole and a darker equator. The equatorial radius of the star was estimated to be 2.03 solar radii, and the polar radius 1.63 solar radii—a 25% increase of the stellar radius from pole to equator.3 Planets the star system has five planets in addition to the dragon homeworld, though three of these are uninhabitable, one of the two, vadimer, is a moon of the dragon homeworld partly responsible for causing the Night of eternal darkness. the other is Casprion, a moon orbiting vadimer.